Known in the art is a method of making semiconductor integrated circuits (cf. U.S. Pat. No. 3,575,741; Cl. HO117/64), also referred to as the collector-diffusion-isolation (CDI) technique, which permits producing circuits containing self-isolated transistors, i.e. transistors in which the isolating region is combined with the collector region. Such circuits are highly advantageous, from the standpoint of the degree of integration, over circuits in which the isolating region is not combined with the collector one and occupies additional space on the wafer.
In this prior art method, a first layer comprising a plurality of n-regions is formed in the surface layer of a semiconductor p-substrate, then an epitaxial p-layer is deposited on the surface of the substrate, which is followed by formation, in the epitaxial layer, of a second layer comprising a plurality of n-regions, with an oxide layer being formed on the surface. Thereafter, a third layer is formed in the surface layer, comprising a plurality of regions of both conductivity types, and the final step is interconnection metallization. According to this method, the third layer is formed as follows. Holes are opened in the oxide layer to form p-regions , then a donor impurity is implanted by diffusion, followed by deep diffusion of this impurity in an oxidizing atmosphere. As a result, another oxide layer is formed on the surface. In the resulting oxide layer, holes are opened to provide for electric contact of different regions of the structure with the metallic interconnections, whereafter the latter are metallized.
After the first oxide layer has been formed on the surface, the deep diffusion in an oxidizing atmosphere is conducted twice, while forming the regions of the second layer, with a new oxide layer being formed twice on the surface. Therefore, the resulting oxide layer underlying the interconnection metallization layer has a varying thickness and features a plurality of steps on its surface. This does not permit forming metallic interconnections (conductors) of small thickness, hence, of small width.
Apart from increasing the width of a metallic conductor, its thickening also results in a longer minimum distance between adjacent conductors. This reduces the degree of integration of a circuit. The high irregularity of the surface, residing in the presence of steps on the oxide layer surface, impairs the circuit reliability.
In addition, conducting deep diffusion in an oxidizing atmosphere necessitates subsequent opening of contact holes. This not only complicates the process of production of a circuit, but also reduces the degree of its integration, for it increases the size of the regions of the third layer because of the necessity to align the holes with respective regions of the third layer.
A semiconductor integrated circuit produced by the prior art method comprises n-p-n transistors and p-resistors. The collector of each n-p-n transistor comprises an n-region of the first layer with an annular n-region of the second layer surrounding it over its perimeter. The collector also serves as a region isolating this transistor from the other circuit components. The base of an n-p-n transistor is formed by a portion of the epitaxial p-layer, confined by the collector region, while the emitter of each transistor is formed by an n-region of the third layer, which is in the base region. The collector contact region is formed by an n-region of the third layer, occupying the annular n-region of the second layer, while the base contact region is formed by a p-region of the third layer, which is in the base region. Each p-resistor is based on a portion of the epitaxial p-layer, confined and isolated from the other circuit components by an n-region of the first layer with an annular n-region of the second layer being provided along its perimeter, the p-region of the third layer, which is formed in the p-resistor region, serving as a region of its contact.
The known integrated circuit lacks such components as p-n-p transistors.
Since a greater number of different components in an integrated circuit permits building more devices on its basis and improving the parameters of these devices, coexistence in the known circuit of n-p-n and p-n-p transistors would have made it possible to create devices with a higher degree of integration and, in some cases, e.g. in integrated circuits of a semiconductor memory, to improve its fast response and power consumption.
Besides, the prior art circuit contains only resistors of a p-type material, which require an isolating n-region occupying additional space on the wafer. This also reduces the degree of circuit integration.